1. Field of the Invention
The present invention relates to a microplate for use in examining samples in the course of clinical examination, DNA analysis and the like.
2. Description of the Related Art
Microplates are vessels widely used in clinical examination, DNA analysis and the like. A microplate has a well, in which a small amount of a liquid sample or a liquid reagent may be contained. There is known such a method that a liquid sample contained in the well of a microplate is subjected to a reaction such as heat cycle or centrifugal separation, a light beam is applied to the liquid sample, and the intensity of the light passing through the sample is measured to determine the results of the reaction. In this method, the composition of the sample and the content of each component thereof can be determined. Inasmuch as a very small amount of a sample or a reagent is required in this method, the method is widely employed to examine blood or urine in diagnosis, to perform DNA analysis, and other clinical examination.
In such a method, it is necessary to divide the same sample into small portions so as to react each of the divided samples with various reagents, respectively, to perform various items of examination. Alternatively, various samples are reacted with the same reagent to conduct one item of examination. To carry out such examination with high efficiency, microplates of the type shown in FIGS. 6A and 6B have been conventionaly used.
FIG. 6A is a side view of a conventional microplate 1, and FIG. 6B is a plan view thereof. The microplate 1 comprises wells 2 and a base 3. The base 3 is substantially rectangular plate. The wells 2 are hollow cylinders, each shaped like a test tube and opening at the top. Each well 2 has an inner wall indicated by the broken line, as shown in FIG. 6A (side view), and can contain a sample. The base 3 has a number of openings that are arranged at regular intervals, in rows and columns. The wells 2 are formed integral with the base 3, each fitted at the bottom in one opening, thus forming the microplate illustrated in FIGS. 6A and 6B. Liquid samples or liquid reagents are dripped into the wells 2 by an automatic distributing apparatus. After the sample or reagent in each cell undergoes a prescribed reaction, it is analyzed by using an optical means.
The any adjacent cells of the same row or the same column are spaced apart by 9 mm. The microplate 1 shown in FIG. 6B has 96 wells 2 (arranged in 8 rows and 12 columns) or 24 wells 2 (arranged in 3 rows and 8 columns) as in most practical cases. Each well 2 has a rim 4 at the upper end. The rim 4 is an annular structure that protrudes a little from the base 3. An adhesive film may be adhered to the rim 4 to prevent evaporation of the sample or cross-contamination of the sample with the sample in the adjacent well 2, which may occur during the analysis of the sample. As shown in the side view (FIG. 6A), the lower end portion of each well 2 is conical, closed at the distal end. Alternatively, it may be cylindrical, having a flat and closed distal end, to serve a specific purpose. The base 3 of the microplate 1 may have side walls that project downwards from the four edges, as is disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 56-115953. The microplate 1 is a molding made of transparent synthetic resin, as described also in Jpn. Pat. Appln. Lain-Open Publication No. 56-115953.
The analysis using the microplates 1 is performed on various samples contained in the wells 2 of the microplate 1. Usually, a plurality of microplates 1 are used, subjecting the samples contained in them to various processes, such as heat cycle and centrifugal separation, which are carried out one after another. Since the samples held in one microplate 1 are different from those held in another microplate 1, it is important to identify any microplate 1 easily. To this end, serial numbers or ID marks are written on the bases 3 of the microplates 1 with felt pens or the like.
It is equally important to identify the sample contained in each well 2 of any microplate. Numbers are therefore printed on a long edge of the microplate 1, indicating the columns of wells 2, and letters a short edge of the microplate 1 to indicate the rows of wells 2, as is illustrated in FIG. 6B. Such a method of identifying the samples is disclosed in Jpn. UM. Appln. Publication No. 5-13399.
As described above, a number or mark is written with felt pens on the base 3 of each microplate 1 to identify each microplate 1 and to identify the wells 2 of the microplate 1. However, the number or mark is liable to erase when the microplate 1 is exposed heat, vapor or organic solvent or accessed to by a person or any instrument during the processes of analyzing the samples contained in the wells 2. If this happens, it will be difficult to identify the microplate 1 and to identify any well 2 with its position on the microplate 1. Most microplates 1 are made of polypropylene resin, because this material excels in heat resistance and for some other reasons. Felt-pen ink can hardly firmly stick to anything made of polypropylene. Therefore, the number or mark written in the ink is easily rubbed off as the rubber-gloved hands touch the base 3 of the microplate 1. In this case, the rubber gloves are stained with the ink. If any person handles the microplate 1 while putting on the ink-stained rubber gloves, the liquid samples may be contaminated. Moreover, when the microplate 1 is exposed to heat, the volatile component of the ink may evaporate, and the number or mark will disappear. The volatile component may be dissolved into the liquid samples, inevitably influencing the results of analysis. Obviously it is troublesome to write the number or mark with felt pens on the base 3 of the microplate 1. If several microplates 1 are stuck one upon another, which often occurs during the analysis, the number or mark written on any microplate 1 laid beneath another can hardly be seen from above. Furthermore, one column number is likely to be taken for another, causing errors in identifying the wells 2, because the column numbers are printed at short intervals on the long edge of the microplate 1.